By Larry Hodgson
You often hear the term “cactus and succulents.” In fact, I use it myself when I write. But I really don’t need to, since all cacti are succulents. So, the term “succulents” should be sufficient. But by using both, I get to avoid having to explain the difference—or lack of difference, depending on the context—between the two.
So, what’s the real situation? Is your plant a succulent? Or a cactus? Or can it be both?
What Is a Succulent?
A succulent plant or simply “succulent” is a plant that has aboveground parts*, either stems or leaves or both, that are more than normally thickened and fleshy in order to store water in arid climates. They “suck up” water during the local rainy season and store it away to ensure their survival during the dry season that follows. A bit like a camel stores water in fat cells in its hump.
*Some definitions of plant succulence also include plants with underground water storage organs.
The word “succulent” comes from the Latin word sucus, meaning juice or sap, because of this characteristic of storing water in the form of sap.
Succulence has evolved independently many times in nature and in many different plant families. In fact, some 65 families of plants have at least a few succulent members. All Cactaceae (cactus), for example, are succulents, as are the majority of Crassulaceae (crassulas, echeverias and sedums) and Aizoaceae (living stones) and a good number of Asphodelaceae (aloes, gasterias and haworthias), Asparagaceae (agaves, beaucarneas, sansevierias and yuccas) and Euphorbiaceae (euphorbias). However, there are succulents even in such unlikely families as the Begoniaceae and the Orchidaceae. There are even succulent morning glories!
Adaptive Evolution in Response to Drought
In response to a climate that is becoming increasingly dry, those plants that are best able to tolerate drought survive and multiply while less drought-tolerant plants are gradually eliminated. When this process is repeated generation after generation, the survival of the fittest starts to operate in a big way, leading to plants that are less and less like their ancestors and more and more able to deal with drought.
Obviously, there are many ways to survive drought. Some plants learn to go dormant when water is scarce. Others reduce the size of their leaves or drop them (most of the water that plants absorb is lost by evapotranspiration through the open stomata of their leaves). Others adopt an annual lifestyle and learn to grow, bloom and go do seed rapidly after a rain before dying. Yet more retreat entirely underground into thickened bulbs, tubers or roots during the dry season, etc.
Storing water reserves above ground in stems or leaves is, however, the most visible way plants face an arid climate.
Many succulent plants, including cacti, abandoned their leaves along the way. These plants learned to carry out photosynthesis using only the chlorophyll-rich green cells in their stems. Thus leaves, which generally lose more water than stems because they’re thinner and have a greater number of stomata, were eliminated. In the case of cacti, those leaves were converted into spines.
Other plants, such as the numerous plants in the Crassulaceae (crassulas, kalanchoes, sedums, etc.) and in the Asphodelaceae (aloes, haworthias, gasterias, etc.) families, kept their leaves, but modified them, covering them with a thick cuticle, reducing the number of stomata and turning them into water storage organs. Their leaves are often covered with white waxy bloom or dense hairs, both of which help reduce water loss by reflecting the sun’s excessively intense rays and reducing air circulation, and thus reduce evapotranspiration.
So, What Are Cactus?
Cactus (the plural can also be cacti) form one family of plants, the Cactaceae, all of whose members are succulents, although a few species are only barely so. And there are a lot of them! Some 127 genera (Echinocactus, Schlumbergera, Echinopsis, Mamillaria, etc.) and over 1700 species, all native to the New World except one.
Therefore, the term “cactus and succulents” is actually redundant. Cacti are succulents, belonging to one of the few plant families whose members are all succulents. So, all the cacti are succulents … but not all succulents are cacti, as there are plenty of leaf and stem succulents in other families.
If you didn’t quite get that all cacti are succulents, but not all succulents are cacti, it’s like poodles and dogs. All poodles are dogs, but not all dogs are poodles.
Cactus Have Spines, Right?
A lot of gardeners think they have it figured out. They know that there are cactus and “other succulents,” but figure the way to tell them apart is the lack of spines of the latter. So, in their definition, if it’s covered in spines, it’s a cactus. If it isn’t, it’s a succulent.
If only it were that simple!
Many “other succulents” have spines. Agaves and aloes are well-known examples. But they do have succulent leaves while cacti are stem succulents. But other unrelated stem succulents too are covered in spines and have no leaves, like many euphorbias, stapelias and huernias. None are related to cacti. Yet others, like the Madgascar palm (Pachypodium lamereri), are covered in spines and seasonally leafless, so may look like cacti part of the year, yet produce dozens of leaves at other times of the year and then look less like cacti.
Add to that the fact that quite a few cacti are spineless, starting with the ever-so-popular Thanksgiving cactus (Schlumbergera truncata), plus orchid cacti (× Epicactus) and the sea urchin cactus (Astrophytum asterias), and the whole “spines on a fat plant equals cactus” theory goes out the window.
You can’t tell a cactus by its spininess!
The Areoles Have It!
The true way to tell cacti from any other succulent is not by their spines, but by their areoles. These are cushiony, fuzzy dots from which spines, stems and flowers grow. They can be white or yellow, sometimes even red, but are present on all cacti. No other plant has areoles.
So, if you look closely at any cactus-like plant and see no areoles, as will be the case with euphorbias and stapelias, that plant is definitely not a cactus! Areoles are, hands down, the easiest way of telling a cactus from any other succulent.
Once you know this, you’ll never confuse a cactus with any other plant again! You have to take a really close look, but it does work.